The present invention generally relates to an optical reader and a light source used for the optical reader, and particularly to an optical reader used in a facsimile machine and a light source including a light emitting diode (LED) array.
A facsimile machine includes an optical system for optically scanning a document.
Referring to FIG. 1, there is shown a mechanism of a facsimile machine, to which the present invention directly relates. A document 13 inserted from a document table 1 is transported in the direction of an arrow A along an upper guide plate 4 and a lower guide plate 5 by document transport rollers 2 and 3. A light source 7 including an LED array is arranged so that light emitted therefrom is projected onto the transported document 13 through an opening window 6 formed in the lower guide plate 5. The light source 7 has a projection optical axis l.sub.1 extending therefrom to the opening window 6. Light emitted from the light source 7 is reflected on the document 13, and passes through a first reflection mirror 8, a second reflection mirror 9 and a third reflection mirror 10 along a read optical axis l.sub.2. Then, the light is converged through an imaging lens 11, and is projected onto a read element 12 which can be a device such as a charge coupled device (hereinafter simply referred to as a CCD).
FIG. 2 is a side view of another optical reader provided in a facsimile machine. In FIG. 2, those parts which are the same as those in FIG. 1 are given the same reference numerals. The document 13 is transported in the direction of an arrow B by the document transport rollers 2 and 3. The light source 7 is positioned so that light emitted therefrom is obliquely projected onto the document 13 through the opening window 6 formed in the lower guide plate 5.
The document 13 is inserted and transported so as to pass between the upper and lower guide plates 4 and 5 by the document transport rollers 2, 3 and pressure rollers 2' and 3'. The document transport roller 2 and the pressure roller 2' are paired, and the document transport roller 3 and the pressure roller 3' are paired. The document transport rollers 2 and 3 are driven by motors (not shown). The pressure rollers 2' and 3' are urged by springs (not shown). The upper guide plate 4 has an area which is painted with a white coating material. This area corresponds to a document read position and faces the light source 7 through the opening window 6. Light emitted from the light source 7 is reflected on the document 13, and passes through a mirror 10' and the imaging lens 11. Then, the light from the imaging lens 11 is projected onto the read element.
A gap W between the upper and lower guide plates 4 and 5 should be between to 0.5-1.5 mm so that the document 13 is transported without jamming. When the gap is made narrower than 0.5 mm, paper jam is liable to occur. In order to prevent the occurrence of paper jam, structural elements must be formed and positioned with high precision. This increases the cost of the device.
When the document 13 passes through the gap W between the upper and lower guide plates 4 and 5, the illuminance is not even but depends on positions of the transported document 13 with respect to the light source 7. FIG. 3 is a graph of an luminance characteristic of a LED. The graph shows that the value of illuminance N decreases with an increase of distance from the center thereof.
FIGS. 4 and 5 are diagrams of conventional optical readers which are directed to overcoming the occurrence of uneven luminance. Referring to FIG. 4, an intersecting point F of the optical axis l.sub.1 extending from the light source 7 and the read optical axis l.sub.2 extending from the document 13 is set at the center of the document passable gap W. A point E on the read optical axis l.sub.2 is closer to the light source 7 than the intersecting point F. However, the point E deviates from a point E.sub.1 on the projection optical axis l.sub.1 by E - E.sub.1. Therefore, the value of luminance (quantity of light) at the point E is approximately equal to or greater than that at the point F. On the other hand, the value of luminance at a point G.sub.1 on the projection optical axis l.sub.1 and the value of luminance at a point G on the read optical axis l.sub.2 decrease as these points go away from the intersecting point F. Therefore, when the document 13 passes between the intersecting point F and the upper guide plate 4, the level of a white video signal output from the read element 12 at this time is not sufficient to indicate white and is thus indicative of gray (i.e., an intermediate color between white and black). That is, an area which is originally to be handled as white is determined to be gray, which deteriorates the quality of a reproduced image.
It is conceivable that the distance G.sub.1 -G is set smaller than the distance E.sub.1 -E, i.e., (G.sub.1 -G)&lt;(E.sub.1 -E) where the optical axis l.sub.1 is arranged so that the intersecting point F is closer to the light source 7 than the point G on the read optical axis l.sub.2. By the above-mentioned arrangement, it becomes possible to obtain almost the same luminance values at the point G (far from the light source 7) and the point E (close to the light source 7). However, the arrangement presents a disadvantage that the luminance value at the point F at the center of the document passable gap W is greater than that at the point E or G, and thus there is a possibility that a gray area on the document 13 may be recognized to be white in error when the document 13 passes through the intersecting point F.
Referring to FIG. 5, the read element 12 is movably provided so as to move in the directions of an arrow M (upper and lower directions) with respect to a read optical axis l.sub.2 ' from the imaging lens 11 (not shown in FIG. 5 for convenience' sake). The position of the read element 12 having the read optical axis l.sub.2 ' is adjusted with respect to the document transport direction K irrespective of the projection optical axis l.sub.1 so that the luminance value at a point I on the upper guide plate 4 is equal to that at a point H on the lower guide plate 5. This causes a substantial positional deviation of the read light axis l.sub.2. As a result, the distance between a document sensor 14 used for determining a read starting position and a document read position is not constant. Thus, it is required to mechanically adjust the read starting position. Unless the read starting position is adjusted, an end area on the document 13 which crosses the projection optical axis l.sub.1 before starting the document read or after terminating the document read, is different for different documents of even same size. Normally, front and rear end portions of the document 13 are handled as blank areas which are not subjected to document reading. If the read starting position is not mechanically adjusted, the blank areas are not constant on an area in the vicinity of the front or rear end position, and thus information to be read may be lost.
FIG. 6 is a diagram of a configuration of the conventional light source 7, and is a graph of an luminance v. distance characteristic of the illustrated light source 7. In the following description, reference "7a" indicates either an LED array or an LED chip. As shown in FIG. 6(a), the light source 7 has an LED array 7a made up of a plurality of LED chips, on both sides of which a pair of reflection frames 15C and 15D are arranged. A rod lens 16 is supported by the reflection frames 15C and 15D. Light beams emitted from the LED array 7a are converged by the rod lens 16 held between the reflection frames 15C and 15D. As shown in FIG. 6(b), the value of luminance N of the light source 7 has a peak on the projection optical axis l.sub.1. The luminance N decreases with an increase in distance from the projection optical axis l.sub.1. If there is an error in position of the LED chip 7a mounted on a base and therefore the LED chip 7a deviates from the projection optical axis l.sub.1 of the rod lens 16, this positional error of the LED chip 7a is increased on the document 13. For example, as shown in FIG. 7, if the optical axis of the LED array 7a deviates from the projection optical axis l.sub.1 of the rod lens 16 by .DELTA.x.sub.1, the optical axis of the deviating LED chip 7a is inclined with respect to the projection optical axis l.sub.1 as shown by a broken line, and a deviation .DELTA.x.sub.2 from the peak equal to a few times the deviation .DELTA.x.sub.1 appears on the document 13. Additionally, an error of the luminance position is caused by deviation in of the rod lens 16 itself and an error in fastening the rod lens 16 to the reflection frames 15C and 15D. A conventional light source configured in a manner similar to that shown in FIG. 5 is disclosed in Japanese Laid-Open Utility Model Application No. 58-177957, Japanese Laid-Open Patent Application No. 57-141174, or 60-147177.
Turning to FIG. 6, the distance y.sub.1 between the LED chip 7a and the document 13 is set long so as to ensure the convergence characteristic of the rod lens 16. For example, the distance y.sub.1 is selected between 10-20 mm. In this case, a difference in quantity of light based on the distance .delta.y between the document 13 and the upper guide plate 4 can substantially be ignored due to the function of the convergence characteristic of the rod lens 16. That is, the illuminance characteristic is not affected by the distance .delta.y.
Alternatively, it is conceivable that a light source is configured without using a rod lens. FIG. 8 is a diagram of a light source which does not use a rod lens. Referring to FIG. 8, a light source 70 includes a printed circuit board 18, on which there are mounted the LED chips 7a forming an LED array, resistors 17 (only one is shown) and a pair of reflection frames 15A and 15B. The printed circuit board 18 is supported by a supporting member 19. The projection optical axis l.sub.1 of the LED chip 7a is inclined with respect to the document 13. With this structure, it is possible to reduce the distance y.sub.2 between the LED chip 7a and the document 13 and the distance y.sub.3 between the LED chip 7a and the upper guide plate 4. Additionally, it is preferable to use the structural elements of small size.
The imaging lens 11 and the read element 12 are positioned so that the document read position corresponds to an luminance position P in the normal direction related to the surface of the document 13. However, the above-mentioned arrangement presents the following disadvantages. There is the difference between the quantity of light V1 on the document 13 and the quantity of light V2 on the white area formed on the upper guide plate 4. This difference is larger than (y.sub.2 /y.sub.3) and less than square of the ratio of y.sub.2 to y.sub.3 as shown below: EQU (y.sub.2 /y.sub.3)&lt;(V.sub.2 /V.sub.1)&lt;(y.sub.2 /y.sub.3).sup.2. (1)
FIG. 9 is a graph of a video signal observed at the document read position P shown in FIG. 8. The illustrated video signal is output from the read element 12 when the document 13 narrower than a document read width R in the main scanning direction perpendicular to the transport direction of the document 13 is set in the facsimile machine 13. In the illustrated video signal, the quantity V1 of light reflected on a portion of the upper guide plate 4 outside of the document 13 is smaller than the quantity V2 of light reflected on the document 13. This is based on the above-mentioned formula (1). As a result, the document 13 is recognized as gray at the time of halftone image processing, for example.